1. Field of the Invention
The present invention relates to an antenna apparatus capable of performing a switching of a directivity pattern.
2. Description of Related Art
Conventionally, it is known that a use of an antenna having no directivity pattern leads to a degradation of communication quality with an interference wave caused by a reflection from a building wall etc. in a multi path propagation environment in which multiple radio waves are available. Thus, an antenna apparatus capable of turning a directivity pattern in a specific direction has attracted attention.
A phased array antenna apparatus shown in FIG. 13 and an adaptive array antenna apparatus shown in FIG. 14 are known as such an antenna apparatus capable of turning a directivity pattern in a specific direction. The phased array antenna apparatus shown in FIG. 13 has N pieces of antenna elements 101-1, 101-2, . . . and 101-N. Then, an amplification of signals having been received by the N pieces of antenna elements 101-1, 101-2, . . . and 101-N is performed by amplifiers (AMP) 102-1, 102-2, . . . and 102-N. The received signals having been amplified by the amplifiers 102-1, 102-2, . . . and 102-N are outputted to a synthesizer 104 after a phase adjustment by variable phase shifters (phase shifters) 103-1, 103-2, . . . and 103-N. The synthesizer 104 performs a synthesis of the received signals from the respective variable phase shifters 103-1, 103-2, . . . and 103-N. A frequency converter (a down-converter) 105 is operated to output the resultant received signal obtained by the synthesizer 104 through a conversion into a signal of a lower frequency.
An adaptive array antenna shown in FIG. 14 has N pieces of antenna elements 111-1, 111-2, . . . and 111-N. In the adaptive array antenna of this type, the amplification of signals having been received by the N pieces of antenna elements 111-1, 111-2, . . . and 111-N is performed by amplifiers (AMP) 112-1, 112-2, . . . and 112-N at the time of a receiving operation of the above antenna. Then, the received signals having been amplified by the amplifiers 112-1, 112-2, . . . and 112-N are respectively down-converted (DC) by frequency converters 113-1, 113-2, . . . and 113-N and subsequently undergo an analog signal-to-digital signal conversion by AD/DA converters 114-1, 114-2, . . . and 114-N. Following the conversion, an output of the obtained digital signals is performed through a so-called adaptive signal processing such as weighting and synthesizing with a digital signal processing unit 115.
On the contrary, at the time of a transmitting operation, digital transmitting signals having been given a required signal processing by the digital signal processing unit 115 are converted into analog transmitting signals with the AD/DA converters 114-1, 114-2, . . . and 114-N and subsequently undergo an up-conversion (UC) with the frequency converters 113-1, 113-2, . . . and 113-N. Following the conversion, the amplification is performed by the amplifiers 112-1, 112-2, . . . and 112-N, leading to a transmission (a radiation) from the antenna elements 111-1, 111-2, . . . and 111-N.
However, the phased array antenna as shown in FIG. 13 requires that a receiving system should be configured with a plurality of variable phase shifters 103-1 to 103-N at a high frequency band. Further, the adaptive array antenna as shown in FIG. 14 requires that the adaptive signal processing should be performed using a plurality of transmitting/receiving systems. For the above reasons, either of the above antenna apparatuses calls for a complicated system and costs much, resulting in a difficult application to a consumer apparatus requiring to be produced at low cost.
By the way, a Yagi-Uda antenna widely used for a reception of television broadcasting is well known as an antenna having a directivity pattern in a specific direction. The Yagi-Uda antenna shown in FIG. 15A comprises a radiator 121 that radiates a radio wave, a director 122 having an electrical length slightly smaller than an electrical length (2/λg, where λg is a guide wavelength) of the radiator 121 and a reflector 123 having an electrical length slightly larger than the electrical length of the radiator 121, wherein the director 122 and the reflector 123 are disposed before and behind the radiator 121 to ensure that the directivity as shown in FIG. 15B is obtained.
Then, a patent document 1 proposes an antenna apparatus that is configured based on the above Yagi-Uda antenna to ensure that a switching of a direction of the directivity is performed. Further, a patent document 2 proposes an antenna apparatus in which a sharing of a director is applied to attain a reduction in antenna size, with reference to an antenna apparatus that performs the switching of a feed point to ensure that a formation of multi-beams is attained. Furthermore, a patent document 3 proposes a multi-beam antenna of multi-frequency sharable type.    [Patent document 1] Japanese Patent Application Publication (KOKAI) No. Hei 11-27038    [Patent document 2] Japanese Patent Application Publication (KOKAI) No. 2003-142919    [Patent document 3] Japanese Patent Application Publication (KOKAI) No. Hei 11-168318